A day after the Swedish army began patrolling the main town on the Baltic island of Jutland, amid rising tensions between NATO and Russia, especially with the recent deployment of a landing ship in the Baltic Sea.

Police in Sweden said they sent patrols and helicopters to the area of the Forsmark nuclear plant, looking for a large marching plane seen flying over the site late Friday evening but could not control it.

Forsmark, Sweden's largest electricity producer, is located on the Baltic coast about 150 kilometres north of Stockholm's capital.

Police spotted the marching plane flying around the station before disappearing towards Grasso Island.

In a report on the incident on its website, the police said, "The police continue to attempt to locate the marching aircraft even by using (their) marching aircraft, but to no avail."

There was no indication that the marching aircraft had dropped anything in the area or landed.

Police also indicated unconfirmed reports of possible sightings of aircraft marching at the Ringgalz nuclear plant on the country's west coast.